In wireless communication networks, an electronic device or station (STA), such as a user equipment (UE), may wirelessly communicate with a network node, such as an access point (AP) or base station (BS), to send data to the network node and/or receive data from the network node.
In a wireless network that uses carrier sense multiple access with collision avoidance, CSMA/CA, an electronic device or network node is configured to listen to the wireless medium and only transmit if the medium is found to be not busy. If the wireless medium is busy, the electronic device or network node must wait until the medium is clear before transmitting. This listening process is referred to as a clear channel assessment (CCA) and is a logical function in the physical (PHY) layer that determines the current state of use of the wireless medium so as to avoid transmission collisions in the medium. The CCA logical function may be based on a CCA threshold, whereby a received signal strength in the wireless medium is compared to the CCA threshold and the wireless medium is determined to be “busy” if the received signal strength meets or exceeds the CCA threshold.
CCA is one of two carrier sense mechanisms commonly used in wireless local area networks (WLANs). For example, CCA is defined in the IEEE 802.11-2007 standards, and the subsequent IEEE 802.11 standards, as part of the Physical Medium Dependant (PMD) and Physical Layer Convergence Protocol (PLCP) layer. CCA involves two related functions: Carrier Sense CCA (CS/CCA) and Energy Detect CCA (CCA-ED).
CS/CCA is a CCA mechanism that is based on measuring the signal strength of a detected WiFi physical layer protocol data unit (PPDU), i.e., relates to the capability of the receiver to detect and decode a WLAN preamble. From the PLCP header field, the time duration for which the medium will be occupied can be inferred and when such WLAN preamble is detected the CCA flag is held busy until the end of data transmission.
CCA-ED is a CCA mechanism that is based on measuring the strength of any received signal, including non-WiFi signals and detected/undetected WiFi PPDUs, i.e., relates to the ability of the receiver to detect received energy in the operating channel and back off data transmission. The ED threshold is typically defined to be 20 dB above the minimum Rx sensitivity of the PHY. If the in-band signal energy crosses this threshold, CCA is held busy until the medium energy is below the threshold.
In the IEEE 802.11 standards, for a primary 20 MHz channel, the CS/CCA threshold is equal to −82 dBm, and a CCA-ED threshold is equal to −62 dBm, which in practice means that any valid signal detected at or greater than −82 dBm shall indicate that the medium is busy, and any energy level detected at or greater than −62 dBm shall also indicate that the medium is busy.
An example of a receiving procedure in an AP or STA that uses CS/CCA in accordance with the IEEE 802.11 standard is shown in FIG. 1. The receiving procedure involves medium access control (MAC) layer and PHY layer functions and includes two states: a CS/CCA state and a Receive (RX) state. In the CS/CCA state a received signal strength indicator (RSSI) measurement is performed by the PHY layer. For example, as generally indicated at 100 in FIG. 1, the PHY layer may measure RSSI during the preamble of a received PPDU. If the RSSI measurement is above a CCAT, the PHY layer sends a PHY-CCA indication to the MAC layer, as generally indicated at 102 in FIG. 1, to indicate that the wireless medium is busy. The PHY-CCA indication is effectively an indication to the MAC layer that a received signal, greater than the CCAT has been received, thus indicating that the medium is busy, and causes the received signal to be processed by the PHY later. During the RX state, the processed signal from the PHY layer is passed to the MAC layer as generally indicated at 104 in FIG. 1. Once the PPDU has been received, decoded and passed to the MAC layer, the PHY layer may then perform a subsequent RSSI measurement and, if the subsequent RSSI measurement is below the CCAT, the PHY layer sends a PHY-CCA indication to the MAC layer, as generally indicated at 106 in FIG. 1, to indicate that the wireless medium is idle.